Integrated circuit dies are conventionally enclosed in plastic or ceramic packages that provide protection from hostile environments and enable electrical interconnection between the integrated circuit die and an underlying substrate such as a printed circuit board (PCB). The leadframe is the central supporting structure of such a package. A portion of the leadframe is internal to the package, i.e., completely encapsulated in the package.
Certain semiconductor varieties present unique packaging needs, such as cavity semiconductors that require a hole for sound or air access to the die. One example of such cavity semiconductors is the micro-electro-mechanical system (MEMS) microphone, which is commonly found in cell phones and like applications. Other types of semiconductor micro-sensors also may have a similar cavity and hole structure.
A typical MEMS silicon microphone has a design based on capacitance variation from diaphragm movement due to sound waves, as depicted generally in FIGS. 1 and 2. The structure of the illustrated MEMS microphone 100 includes an etched cavity 110, a moving plate or diaphragm 120, and a fixed plate 130. FIG. 1 illustrates a variation of the design (Type A) of the MEMS microphone 100 with the diaphragm on the bottom side of the microphone die. FIG. 2 shows another design variation (Type B) of the MEMS microphone 200 with the diaphragm 120 on the top side of the die. When assembled into a cavity package, the package can have an acoustic port placed either at the package's top side (for the type B design) or on its bottom side (for type A). This preferred placement of the acoustic port allows the sound wave to impinge directly on to the diaphragm rather than having the sound pass through the plate.
Current approaches to packaging cavity semiconductors include using ceramic packages, or plastic packages with laminate substrate, or a pre-molded leadframe in which the lid or base has a hole.
Ceramic packages have been used for MEMS applications in which the integrated circuit die cannot be capped. However, ceramic packages are expensive compared to plastic packages. For this reason, the semiconductor industry wherever possible has moved away from the expensive ceramic packaging.
Another approach for MEMs microphone packaging uses a plastic package 300 with a laminate substrate 310, such as illustrated in FIG. 3. This package encloses the MEMS microphone die 200 along with an application-specific integrated circuit (ASIC) chip 320, both supported on the substrate 310. The package may also contain other discrete components, such as surface-mount technology (SMT) capacitors. The substrate is a laminate printed circuit board (PCB) variety. The package includes a lid 330, which may be a single piece construction or multiple pieces. The lid has an acoustic opening or port 340, and along with the laminate substrate defines an acoustic cavity 350 containing the dies 200, 320.
A laminate substrate similar to that in the package shown in FIG. 3 also is commonly used in BGA (Ball Grid Array) or other array packages. However, laminate substrates also contribute significantly to the expense of the package.
An alternative approach for further reducing cost is to use a pre-molded leadframe. FIG. 4 illustrates one example of a pre-molded leadframe package 400 for a MEMS microphone. The package 400 includes a pre-molded leadframe 410 and a lid 420, which together define an acoustic cavity 450 containing the MEMS microphone die 200. The lid again may be a single piece construction or multiple pieces, and has an acoustic port 440 in the lid. By using a pre-molded leadframe instead of a laminate substrate, the cost of the package is lower compared to a package with laminate substrate.
It is known that size or location of the acoustic port 340, 440 and cavity 350, 450 of the package 300, 400 affect the microphone's performance. Thus, a proper package design that takes into account the configuration of the MEMs microphone chip, the package acoustic port and cavity is critical for performance. In addition, the path from sound source to package port 340, 440 for a particular application preferably is short.
In the case of a cell phone application for example, the MEMS microphone package preferably has its acoustic hole on the bottom side of the package. FIG. 5 illustrates a typical configuration of a cell phone 500 using the pre-molded leadframe package 400. The cell phone handset is configured to have the microphone near the bottom of the keypad, so as to be near the mouth when the handset is held to the user's ear. The case 510, 511 of the cell phone therefore has a hole or holes 530 on its front side 510, next to the keypad (not shown). The electronic components of the cell phone 500 are supported on a printed circuit board (PCB) 540 inside the case 510. As with most thin cell phone designs, the electronic components 550 of the phone are on a back side of the PCB 540, opposite to the keypad. Except, the microphone package 400 is placed on the front side of the PCB 540 to provide a short acoustic path to the microphone die. The microphone's performance is further enhanced with the addition of a gasket 560 between the microphone package 400 and front side 510 of the case. The gasket can be a flat rubber piece with a hole.
The microphone package design in this typical cell phone configuration has some disadvantages. First, the placement of the microphone package on the front side of the PCB increases the cell phone thickness. Further, the gasket between the microphone package 400 and case can potentially cause stress on the microphone package.
For these reasons, a cell phone design 600 shown in FIG. 6 that reflows the microphone package along with the other components 550 onto the back side of the PCB 540 is preferred. For this cell phone design, the microphone package 700 (shown in FIG. 7) is the laminate substrate type, with a hole 730 formed through the laminate substrate 720 on a bottom side of the package 700. The PCB 540 also has a corresponding hole 640. The microphone package also provides an acoustic cavity 750 formed in the laminate substrate 720. This configuration of the microphone package to have a bottom port and acoustic cavity allows designers to reflow the microphone package to the PCB back side and thereby achieve a much thinner cell phone design. However, the design still has disadvantages: (1) the laminate substrate remains more expensive compared to a pre-molded leadframe, and (2) the acoustic cavity inside the laminate substrate increases the thickness of the substrate, and requires more layers of laminate stack-up for the substrate.